In the past, computers were used only by scientists, mathematicians, and other high-level, sophisticated computer users. As computer technology progressed, and particularly with the advent of the personal computer, data processing has reached every level of society, and every level of user. The trend is for fewer computer users to be computer professionals or sophisticated in data processing techniques. Access to computers will increase even further in the future as computer hardware and software increase in power and efficiency.
It has therefore become necessary to design what have become known in the art as "user friendly" input devices. Such "user friendly" devices are designed to allow an unsophisticated user to perform desired tasks without extensive training. Human factor studies have shown that a device which allows the user to input data directly on the visual display screen of a computer, generally known in the art as a touch input device, achieves greatest immediacy and accuracy between man and machine. One of the first input devices for use at the display surface was the light pen. The light pen is an optical detector in a hand held stylus, which is placed against the face of a cathode ray tube. The location of the light pen is determined by detecting the coordinates of the dot of light which is the scanning raster of the display. A second type of touch input device is a mechanical deformation membrane which is placed over the display screen. The membrane is a transparent overlay which consists of two transparent conductor planes disposed on a flexible surface. When a selection is made, the user mechanically displaces one of the conductor planes to touch the other by a finger or stylus touch, thereby bringing the conductors into electrical contact with each other. Appropriate electronics and software translate the electrical signals generated by the finger or stylus touch to the position on the display surface. Another touch input device is a capacitive transparent overlay placed over the display screen, which includes transparent conductors driven by an electromagnetic signal. The input device can detect the location of a finger touch by the change in capacitance of the overlay or, alternately, a stylus is used to return the electromagnetic signals from the overlay back to the computer to determine the stylus position. Yet another touch input device uses a frame which fits around the display screen having a number of infrared or visible light transmitters and receptors arranged in parallel horizontal and vertical directions. When the user's finger blocks the light beams, the horizontal and vertical receptors note the absence of the signals, thereby locating the position of the action desired by the user.
As such touch input devices have proliferated, there have been many efforts to write user friendly software as well. Recently, graphical user interfaces which have the user point to the screen to select objects and initiate actions have become popular. These graphical user interfaces typically present choices to the user in menu or window form. In addition, recent computer applications use stylus devices for freehand drawing, gesture recognition, and handwriting capture. These new software applications utilize the capabilities of the touch input devices to emulate the familiar ergonomics of a paper and pen to input data into a data processing system. These stylus applications require more precise detection of stylus contact with the screen than do other applications.
A particularly versatile touch input system is described in U.S. Pat. No. 4,686,332, to E. Greanias, et al., entitled "Combined Finger Touch and Stylus Detection System for Use on the Viewing Surface of a Visual Display Device" filed Jun. 26, 1986 which is hereby incorporated by reference. For certain applications, such as selecting items from a list, finger sensing methods have been found more convenient. Where greater precision is required, such as applications with a high information density, or where freehand drawing or handwriting is recognized, the use of a stylus has been found more effective. The touch input system described in the above referenced U.S. Pat. No. 4,686,332 allows for both finger touch and stylus detection. The system includes a touch overlay sensor which comprises an array of horizontal and vertical transparent conductors arranged on the viewing surface of the visual display device. The conductor array emits electromagnetic signals into the region above the display surface under the control of a microprocessor. The magnitude of these signals is greatest near the surface and grows smaller at greater distances. A stylus "antenna" is connected to an input of the detector control system and senses the signals emitted by the array. The signal amplitude seen by the stylus is related to the position of the stylus on and above the display. Radiative signal measuring means coupled to the stylus measures the electromagnetic or electrostatic signal received by the stylus. Stylus contact with the display surface is indicated when the electromagnetic signal exceeds a prescribed threshold. The accuracy of contact determination depends on the uniformity of the radiated signal across the touch overlay surface.
The system includes a means for connecting the output of an electromagnetic or electrostatic radiation source to selected patterns of horizontal and vertical conductors in the array. A switchable path connected to the I/O terminals of the array selects the plurality of horizontal and vertical conductors that are connected to the radiative source. Control signals applied to the control input of the switchable path determine the conductors that are connected at different intervals of the sensing procedure. The control signal timing is used to interpret the stylus signal amplitude and determine where the stylus is located with respect to the conductor array in the plane of the display surface. The finger sensing system in U.S. Pat. No. 4,686,332 is also a capacitive measurement means which measures the capacitance of selected conductors and determines where and when a finger touch occurs. The same switchable path is also used to connect the capacitance measurement means to pluralities of horizontal and vertical conductors to the capacitance sensing means in response to control signals applied to the control input.
However, the system as described in the U.S. Pat. No. 4,868,332 has a number of drawbacks, particularly with regard to the detection of stylus contact with the sensor screen. For handwriting applications, only the positions of the stylus as it touches the sensor screen should be recorded. For example, the stylus motion after completing the stem of a "t", and before the crossing is begun, must not be recorded, even if the stylus is moved very near the surface of the sensor. Similarly, the stylus motion between the horizontal lines of an equal sign must not be recorded even if the stylus moves very near the surface. With the sensing method as described in U.S. Pat. No. 4,686,332, the stylus would frequently be in sufficient proximity to the touch overlay for positions between strokes to be recorded in the tracking mode. Thus handwriting recognition would have a great number of unintended strokes.
As discussed in the U.S. Pat. No. 4,868,332, the conductors in the touch overlay are approximately 0.025 wide and are spaced approximately 0.125 inches center-to-center. When compared to the resolution desired for handwriting, on the order of 250 points per inch, this is a relatively wide spacing. To determine the position of the stylus when it was between adjacent conductors, an interpolation technique was used. This technique assumed that the field varied linearly with position between the two conductors where the second set of three conductors was connected to ground. While this assumption was to determine the stylus position with more than a fair degree of accuracy, it is not true. As the electrical field strength from an individual conductor varies with the distance from the individual conductor, dielectric properties of the materials surrounding the conductor and the location of nearby grounded conductors, the electrical field strength from the multiple driven wires exhibited non-linear characteristics. This effect is more pronounced where the layers in the touch overlay are thin, and thus, the stylus is closer to the individual conductors interpolation technique used.
Lastly, it was found that the attitude of the stylus itself as it was held in the hand of the user had an effect on the location sensed by the system. As mentioned above, the stylus acts as an antenna to pick up the electromagnetic signals radiated by the touch sensor overlay. Depending on the writing angle preferred by an individual user, the signal strengths measured by the system could vary considerably, thus creating errors in the accurate locating of the stylus. Contrary to the assertions of the U.S. Pat. No. 4,686,332 in column 6, lines 56-60, the signal strengths can not always be normalized by calculation, as the stylus orientation can change during the stroke across the overlay.